Cascaded thyristor quenching arrangement for a pulsed flash device

ABSTRACT

A compact, high-speed quenching arrangement is described for use with a portable flash unit that is required to handle moderately high peak currents during quench. A pair of normally nonconductive cascaded thyristors are connected in shunt with the flash tube. When a predetermined quantity of emitted light has been monitored and integrated by the flash unit, the first thyristor is switched into conduction. The resulting build-up of current in the transconductive path of the first thyristor is converted to a proportional control voltage which triggers the second thyristor into conduction to handle the portion of the quench load that exceeds the capacity of the first thyristor. Protective resistors in the respective transconductive paths of the thyristors may be suitably selected to accommodate a range of peak current conditions of the flash device.

I United States Patent [151 3,662,21 3 Dennewitz et al. 1 May 9, 1972 [54] CASCADED THYRISTOR QUENCHING 3,293,449 12/1966 Gutzwiller ..307/252 K ARRANGEMENT FOR A PULSED 3,509,422 4/1970 Kilgore ..3l5/l5l D I FLASH Ev CE Primary E.\'aminerRoy Lake [72] Inventors: Rolf Dieter Dennewitz; Emil A. Exner; Assistant Examiner-Lawrence .l. Dahl Henning Zierau, all of Berlin, Germany AttorneyF. R. Trifari [73] Assignee: U.S. Philips Corporation, New York, NY. [57] ABSTRACT [22] Filed: 1970 A compact, high-speed quenching arrangement is described [2l] Appl. No.: 8,271 for use with a portable flash unit that is required to handle moderately high peak currents during quench. A pair of normally nonconductive cascaded thyristors are connected in Foreign Application Prlomy Data shunt with the flash tube. When a predetermined quantity of June 3 1969 Germany "I9 28 1573 emitted light has been monitored and integrated by the flash unit, the first thyristor is switched into conduction. The resultlsz} US Cl i I A i m3l5/l49 315/24 ing build-up of current in the transconductive path of the first 511 im. Cl. "Bosh 37/02 thyriso is a Propommal comm "wage which [58] Field g 315/149 159 H9 24] P triggers the second thyristor into conduction to handle the 174 3'07/2'52 252 6 portion of the quench load that exceeds the capacity of the first thyristor. Protective resistors in the respective transconductive paths of the thyristors may be suitably selected to ac- [56] References Cited commodate a range of peak current conditions of the flash UNITED STATES PATENTS device 3,541,387 1 H1970 Ackermann ..3 15/151 10 Claims, 1 Drawing Figure /3 0.6. 4/! POWER M (./'T 70 .1 RC IGNITION SOU E X FORMER /5 BACKGROUND OF THE INVENTION In recent years various devices employing light integrating means have been disclosed for automatically adjusting the operating duration of a pulsed flash device. Such devices employ a gas discharge tube in shunt with the flash tube to quench the flash when the integrated light derived from the emission of the flash tube has reached a predetermined value. Examples of such adjusting arrangements are described, e.g., in US Pat. No. 3,033,988 issued to H. E. Edgerton and in US. Pat. Nos. 3,350,603 and 3,350,604 issued to R. D. Erickson.

Such gas discharge quench tubes are not generally suitable for use in portable flash devices since they are bulky in themselves and require heavy-duty ignition circuitry and electrostatic shielding to prevent premature ignition. Additionally, their switching speed is relatively low.

One type of improved quenching arrangement for alleviating these problems is described in the copending application Ser. No. 882,689 entitled High Speed Quenching Arrangement for a Flash Device." In that arrangement, the conductive state of a single normally disabled thyristor connected in shunt with the flash tube is controlled by the collector potential of a threshold-regulated transistor that is driven into conduction when the flash tube starts to emit light energy. During conduction of the transistor, its collector potential is insufficient to drive the thyristor into conduction. However, when the control signal developed in the light integrating portion of the flash unit has reached a value sufficient to overcome a predetermined threshold level at the input of the transistor, the latter is switched off and the associated rise in potential at its collector triggers the thyristor into conduction to short-circuit the flash tube and terminate the flash.

It is found that when this type of arrangement is employed in portable flash units that must operate with relatively high peak current loads, the quenching current surge through the single shunting thyristor, being concentrated in the immediate vicinity of the thyristor control electrode, may render the device susceptible to bum-out even when a current-limiting resistor is included in its transconductive path.

SUMMARY OF THE INVENTION In order to preserve the high switching speed and other advantages of the thyristor quench arrangement principles described in the copending application while at the same time accommodating relatively high currents during quench, the present invention may be advantageously employed.

In an illustrative embodiment, the transconductive paths of a pair of normally disabled cascaded thyristors are connected across the flash tube through current-limiting resistors. The control electrode of the first thyristor is connected to the output of a light-integrating arrangement that is rendered effective at the start of the flash for generating a control signal proportional to the integrated light energy contained in the flash tube emission. When the control signal has reached a value sufficient to trigger the control electrode of the first thyristor, the latter starts to conduct to bypass a portion of the flash capacitor energy from the flash tube. An additional resistor in series with the first thyristor responds to the resulting flow of current therethrough to develop a voltage that is applied to the control electrode of the second thyristor so that the latter is instantaneously switched into conduction to absorb the remaining flash capacitor energy. The total switching time between the triggering of the first thyristor and the full conduction of the second thyristor may be as low as l microsecond.

The peak current load that may be handled by the cascaded thyristor quenching arrangement may be adjusted by suitably selecting the magnitude of the current-limiting resistors in the transconductive paths of the thyristors.

BRIEF DESCRIPTION OF THE DRAWING The nature of the invention and its advantages will appear more fully from the following detailed description taken in conjunction with the appended drawing, in which the single FIGURE depicts a high-speed, high-capacity quenching arrangement in accordance with the invention.

DETAILED DESCRIPTION Referring now to the drawing, there is shown a pulseexcited flash unit including a conventional flash tube 1 (illustratively a gas filled envelope). The tube 1 is provided with a pair of excitation terminals 21 and 22. A primary winding 2a of a transformer 2 is connected between the terminal 22 and a common terminal 23 of the flash unit. A flash capacitor 14 is connected across the terminals 21 and 23.

The capacitor 14 may be selectively charged in a known manner to a relatively high potential of the polarity shown from a conventional DC power source 15.

The flash tube 1 is further provided with a trigger electrode 13. While not specifically illustrated in the drawing, the trigger electrode may be coupled to the secondary winding of a conventional ignition transformer whose primary winding may be excited by a synchronizing pulse in a known manner.

The voltage of the charged flash capacitor 14 is not normally sufficient by itself to cause emission of light energy from the tube 1. However, as is well known, this tube will emit light upon the concurrent application of an igniting pulse to the trigger electrode 13 through its associated ignition transformer. The resulting flash of light energy produced in the tube 1 is emitted through the walls of the tube envelope to a suitable object (not shown). A portion of the light reflected from the object is detected by a photosensitive element 5 (illustratively a light sensitive resistor) which is connected in a light integrating circuit 24 in the manner described below.

When the required coincidence occurs between the igniting pulse voltage on the electrode 13 and the flash capacitor voltage across the terminals 21 and 23, the resultant conduction of the flash tube: (A) causes light energy to be emitted toward the object to be illuminated, and (B) causes a pulse of current to pass through the primary winding 2a of the transformer 2. A corresponding voltage pulse is thereby generated across a secondary winding 2b of the transformer 2. Such voltage pulse is coupled through a rectifying diode 3 to establish a voltage across a capacitor 4.

A voltage divider consisting of the light sensitive resistor 5 and an additional pair of resistors 6 and 7 (the former being adjustable) is connected across the capacitor 4. The resulting potential appearing at a junction 26 of the resistors 6 and 7 (which junction forms the output of the light integrating circuit 24) is coupled, through a diode 8, to the control electrode of a first normally disabled thyristor 9. The transconductive path of the thyristor 9 is connected through a pair of resistors 10 and 13' across the terminals 21 and 23 for shunting the flash tube 1. Such transconductive path is therefore excited whenever the flash capacitor 14 is in its charged state.

In accordance with the invention, a second normally disabled (cut-off) thyristor 12 is connected in cascade with the thyristor 9 across the flash tube 1 to absorb a portion of the flash capacitor energy bypassed from the flash tube 1 during a quenching operation, as described below. To this end, a junction of the resistor 10 and the cathode of the first thyristor 9 is connected through a second isolating diode 11 to the control electrode of the second thyristor 12. The transconductive path of the the thyristor 12 is connected, through a resistor 14', across the terminals 21 and 23 so that the thyristor 12 is also normally energized by the voltage on the flash capacitor 14. The characteristics of the thyristors 9 and 12 and the resistors 10, 13, and 14 may be selected such that the thyristor 12 is immediately triggered into conduction by the voltage developed across the resistor 10 by the current resulting when the thyristor 9 is driven into conduction.

Such conduction of the thyristor 9 is initiated by the action of the light integrating circuit 24. In particular, when the flash tube 1 is triggered into conduction, the voltage on the capacitor 4 builds up at a rate determined by variations in the resistance of the light sensitive resistor in the shunting voltage divider. Such variations are in turn proportional to the intensity of the light reflected to the resistor 5 from the illuminated object.

With this arrangement, the voltage across the capacitor 4, and therefore the proportional voltage at the junction 26, is indicative of a quantity proportional to the total integrated reflected light from the object. The thyristor 9 will conduct when the potential at the junction 26 has built up to a point that exceeds the characteristic threshold triggering level at the control electrode of the thyristor 9.

The operation of the above-described arrangement is as follows:

Prior to the start of light emission from the tube 1 (as when no excitation appears on its trigger electrode 13), no current flows through the primary winding 2a of the transformer 2. Thus no voltage is developed across the secondary winding 2b and the capacitor 4 is at zero potential. The corresponding zero potential at the junction 26 maintains the initially nonconductive thyristors 9 and 12 in their disabled states, and both thyristors will appear as open circuits across the flash tube 1.

When a pulse is applied to the trigger electrode 13 of the flash tube 1 through its ignition transformer to cause the tube to emit light, the resulting pulse of current through the primary winding 2a will result in a pulse of charging current applied to the capacitor 4 through the secondary winding 2b and the diode 3. The voltage across the capacitor 4 thereupon builds up in a manner determined by the characteristic of the light sensitive resistor 5 in response to the light reflected thereto from the object. The resulting potential at the junction 26 builds up in proportion to the total light energy integrated by the circuit 24.

When the emission of light from the tube 1 has persisted for a duration long enough to permit the voltage at the control electrode of the thyristor 9 to exceed its threshold trigger value, the thyristor is rapidly driven into conduction to efiectively present a low impedance across the flash tube 1. A portion of the energy from the flash capacitor 14 is thereupon bypassed from the flash tube 1 to the now-conductive thyristor 9 to commence the quenching operation. The resulting surge of current through the transconductive path of the thyristor 9 causes a proportional voltage to be developed across the resistor 10. Such voltage, applied to the control electrode of the second thyristor 12 through the diode l 1, drives the thyristor 12 into conduction to further lower the total shunting impedance across the tube 1 thus to absorb the remaining bypassed energy from the flash capacitor 14, thereby preventing overloading of the thyristor 9. Because of the rapid switching action of the thyristors compared to a gas discharge tube, the total interval between the triggering of the first thyristor 9 and the full conduction of the second thyristor 12 can be as low as l microsecond and even lower.

When the flash capacitor 14 has discharged through the shunting thyristors 9 and 12 for an interval sufficiently long such that the flash capacitor voltage is no longer sufficient to maintain the transconductive paths of the thyristors energized, the thyristors will a assume their nonconductive states to again present open circuits across the flash tube 1.

The quantity of light energy emitted by the flash unit before quench may be regulated, e.g., by suitably varying the value of the adjustable resistor 6 in the light integrating circuit 24. Also, it will be understood that the current load handled by the thyristors 9 and 12 may be varied by suitably selecting the magnitudes of the current limiting resistors 13' and 14'.

From the previous description it will be appreciated that, like the single quenching thyristor described in the abovementioned copending application, the cascaded thyristor arrangement of the instant invention avoids the problem of large and bulky gas discharge devices with their accompanying heavy-duty ignition circuitry and electrostatic shields. in addition, because of the high switching speed of the thyristors, the associated flash unit may safely operate with objecrto-flashlamp distances so small as to normally cause excessive exposure of the object to be illuminated when prior art gas discharge quench tubes are used.

Although the invention has been illustrated and described with reference to the preferred embodiment thereof, it is understood that it is in no way limited to the details of such embodiment but is capable of numerous modifications within the scope of the appended claims.

What is claimed is:

1. An apparatus for rapidly terminating the output of a pulsed flash tube when a controllable quantity of light energy has been emitted therefrom comprising:

first means rendered effective at the start of the flash for generating a control signal that varies in proportion to the light energy emitted from the flash tube;

first and second normally disabled thyristors having their respective transconductive paths connected in parallel across the flash tube;

first means for coupling the output of the first generating means to the control electrode of the first thyristor for triggering the latter into conduction when the control signal has reached a predetermined value;

second means responsive to current flow through the first thyristor during conduction thereof for generating a proportional voltage of a magnitude sufficient to trigger the second thyristor into conduction; and

second means for coupling the output of the second generating means to the control electrode of the second thyristor.

2. Apparatus as defined in claim 1, in which the first generating means comprises, in combination, a capacitor, a transformer having a primary winding in series with the flash tube and a secondary winding connected across the capacitor, and a voltage divider including a photosensitive element connected across the secondary winding and the capacitor.

3. Apparatus as defined in claim 2, in which the first coupling means comprises a diode connecting a tap point of the voltage divider to the control electrode of the first thyristor.

4. Apparatus as defined in claim 1, further comprising a pair of current-limiting resistors individually connected in the transconductive paths of the first and second thyristors, respectively.

5. Apparatus as defined in claim 4, in which the second generating means comprises an additional resistor connected in the transconductive path of the first thyristor.

6. A flash tube ignition control circuit comprising, a capacitor connected across the flash tube for supplying operating current thereto, means coupled to the flash tube for igniting said flash tube, first and second substantially identical normally nonconductive load sharing semiconductor controlled conduction devices each having a rated current capacity which is insufficient by itself to safely pass the normal expected operating discharge current of said capacitor during a tube quenching operation, means connecting said first and second semiconductor devices in parallel across the flash tube, a light integrating circuit responsive to the flash tube current for generating a control signal upon ignition of the flash tube, said integrating circuit including a photosensitive element arranged to respond to a portion of the flash tube light reflected by an object thereby to cause said control signal to vary as a function of said reflected light, means for coupling said control signal to the control electrode of the first semiconductor device to trigger same into conduction to initiate the quenching operation when the control signal attains a predetermined value, means responsive to the current flow through the first semiconductor device for generating a voltage of a magnitude to trigger the second semiconductor device into conduction, and means for coupling the trigger voltage to the control electrode of the second semiconductor device to immediately trigger same into conduction upon conduction of the first semiconductor device thereby to share the discharge load current of said capacitor between said semiconductor devices.

7. An ignition circuit as claimed in claim 6 wherein said integrating circuit further comprises, a second capacitor and means responsive to the flash tube current for supplying a charge current to the second capacitor that is a function of the flash tube current.

8. An ignition circuit as claimed in claim 7 wherein said integrating circuit further comprises means for electrically isolating said second capacitor from any electrical energy source so that the second capacitor voltage returns to approximately zero value in the absence of current flow through the flash tube.

9. An ignition circuit as claimed in claim 7 wherein said charge current supply means comprises a transformer with a primary winding in series with the flash tube and a secondary winding connected across said second capacitor.

10. An ignition circuit as claimed in claim 9 wherein said integrating circuit further comprises a resistive voltage divider connected in series with said photosensitive element across the terminals of said second capacitor, and said igniting means comprises an igniter electrode for the flash tube that is energized independently of said integrating circuit. 

1. An apparatus for rapidly terminating the output of a pulsed flash tube when a controllable quantity of light energy has been emitted therefrom comprising: first means rendered effective at the start of the flash for generating a control signal that varies in proportion to the light energy emitted from the flash tube; first and second normally disabled thyristors having their respective transconductive paths connected in parallel across the flash tube; first means for coupling the output of the first generating means to the control electrode of the first thyristor for triggering the latter into conduction when the control signal has reached a predetermined value; second means responsive to current flow through the first thyristor during conduction thereof for generating a proportional voltage of a magnitude sufficient to trigger the second thyristor into conduction; and second means for coupling the output of the second generating means to the control electrode of the second thyristor.
 2. Apparatus as defined in claim 1, in which the first generating means comprises, in combination, a capacitor, a transformer having a primary winding in series with the flash tube and a secondary winding connected across the capacitor, and a voltage divider including a photosensitive element connected acRoss the secondary winding and the capacitor.
 3. Apparatus as defined in claim 2, in which the first coupling means comprises a diode connecting a tap point of the voltage divider to the control electrode of the first thyristor.
 4. Apparatus as defined in claim 1, further comprising a pair of current-limiting resistors individually connected in the transconductive paths of the first and second thyristors, respectively.
 5. Apparatus as defined in claim 4, in which the second generating means comprises an additional resistor connected in the transconductive path of the first thyristor.
 6. A flash tube ignition control circuit comprising, a capacitor connected across the flash tube for supplying operating current thereto, means coupled to the flash tube for igniting said flash tube, first and second substantially identical normally nonconductive load sharing semiconductor controlled conduction devices each having a rated current capacity which is insufficient by itself to safely pass the normal expected operating discharge current of said capacitor during a tube quenching operation, means connecting said first and second semiconductor devices in parallel across the flash tube, a light integrating circuit responsive to the flash tube current for generating a control signal upon ignition of the flash tube, said integrating circuit including a photosensitive element arranged to respond to a portion of the flash tube light reflected by an object thereby to cause said control signal to vary as a function of said reflected light, means for coupling said control signal to the control electrode of the first semiconductor device to trigger same into conduction to initiate the quenching operation when the control signal attains a predetermined value, means responsive to the current flow through the first semiconductor device for generating a voltage of a magnitude to trigger the second semiconductor device into conduction, and means for coupling the trigger voltage to the control electrode of the second semiconductor device to immediately trigger same into conduction upon conduction of the first semiconductor device thereby to share the discharge load current of said capacitor between said semiconductor devices.
 7. An ignition circuit as claimed in claim 6 wherein said integrating circuit further comprises, a second capacitor and means responsive to the flash tube current for supplying a charge current to the second capacitor that is a function of the flash tube current.
 8. An ignition circuit as claimed in claim 7 wherein said integrating circuit further comprises means for electrically isolating said second capacitor from any electrical energy source so that the second capacitor voltage returns to approximately zero value in the absence of current flow through the flash tube.
 9. An ignition circuit as claimed in claim 7 wherein said charge current supply means comprises a transformer with a primary winding in series with the flash tube and a secondary winding connected across said second capacitor.
 10. An ignition circuit as claimed in claim 9 wherein said integrating circuit further comprises a resistive voltage divider connected in series with said photosensitive element across the terminals of said second capacitor, and said igniting means comprises an igniter electrode for the flash tube that is energized independently of said integrating circuit. 